The present invention relates to a centrifugal compressor having an impeller.
A centrifugal compressor is known as one of compressors for compressing gas. Japanese unexamined patent publication No. 2005-194933 discloses a centrifugal compressor which has a fluid passage for communicating with a diffuser, a pair of scrolls for receiving gas from the diffuser and discharging out of the compressor, and a changing means for changing the width of the passage in the diffuser. The changing means changes the width of the passage so that the passage is changed into a narrow passage state where the diffuser is in communication with only one of the scrolls, or into a wide passage state where the diffuser is in communication with both of the scrolls.
The above-described centrifugal compressor has a first operational mode and a second operational mode, which are set selectively. The first operational mode is set so that the width of the passage in the diffuser is widened to utilize two scrolls. The second operational mode is set so that the width of the passage in the diffuser is narrowed to utilize one scroll. That is, in the centrifugal compressor, the first operational mode and the second operational mode are set alternatively by switching the state between the narrow passage state and the wide passage state. Accordingly, the centrifugal compressor can achieve high compression efficiency in a substantially wide range of the flow rate with its simple structure.
The above-described compressor can set the width of the passage in the diffuser variably by the changing means for changing the width of the passage. However, diffuser stall still exists continuously. When the flow rate in the centrifugal compressor is extremely low, specifically, diffuser stall occurs inevitably in the compressor. Diffuser stall invites problems such as vibration of the compressor, and thereby prevents the stable operation of the compressor.
The present invention is directed to providing a centrifugal compressor in which diffuser stall is prevented when the flow rate of the gas is low, so as to obtain stable operation in a wide range of the flow rate.